Five in all, two when Kirk is attacked by Chekov, and three vapourised by Moreau with the Tantalus device. Kyle and Chekov are both tortured, but survive.

Factoid :

This episode features the first use of the mirror universe which will be revisited several times during Deep Space Nine.

The Mirror Sulu wears a red uniform in this episode. Since George Takei has previously worn a blue uniform in "Where No Man Has Gone Before" as well as his normal Gold, this makes Takei the first actor ever to wear all three uniform colours.

At one point Scotty refers to Kirk as "Jim" - the only time Scotty ever referred to the Captain that way to his face.

This episode was nominated for the 1968 Hugo Award for "Best Dramatic Presentation".

The idea of evil counterparts from an alternate universe is one that became a common trope in fiction, often including the idea that evil counterparts should have a goatee beard. South Park parodied the idea by having the counterpart of our universe's Cartman being the good one.

Plotline

The Enterprise is at the planet Halkan to negotiate for the right to mine dilithium crystals - unfortunately the ultra pacifistic Halkans refuse to grant permission to mine their planet under any circumstances, lest the crystals be used for violent purposes. As the landing party prepares to beam up an ion storm engulfs the planet, rocking the Enterprise in orbit. As the transporter engages a strange effect sweeps Kirk, McCoy, Scotty, and Uhura onto a strangely different Enterprise.

Some of the changes are small - uniform designs, the decoration of the ship. But others are more serious - First Officer Spock responds to a perceived failure on the part of the transporter operator by using a small device on his uniform to torture him, then decalres casually that the Halkan's refusal to negotiate necessitates the usual response of genocide.

Thinking fast, Kirk and the others play along with whatever has happened as they try to learn about their situation. It soon becomes clear that this is the "ISS" Enterprise, a warship of the Terran Empire. The Empire is a moral inversion of the Federation - a brutal dictatorship bent on the conquest and destruction of other races. Things are little better within; genuine loyalty and camaraderie is completely absent from the crew, instead replaced by intense competition and one-upmanship. Crewmembers are constantly seeking to assassinate their superiors in order to move up in rank, and casual cruelty is the norm - McCoy discovers doctors in sickbay torturing patients whilst betting on how much pain they can withstand.

The officers wonder if the use of the transporter during the ion storm may have catapulted them into some alternate universe. They begin to search for a way back to their own Enterprise whilst trying to avoid the cruelties expected of them. Uhura determines that Captain Kirk's orders leave no room for compromise with the Halkans, whilst Scotty is unable to sabotage the phasers because they are guarded by security officers. Kirk declines to fire on the Halkan cities in violation or orders, saying he wants to continue negotiations. When they refuse he gives them 12 hours to reconsider, much to Spock's surprise, and then departs the bridge.

As Kirk makes his way through the ship he is ambushed by a group led by Chekov who try to kill him. Chekov gloatingly tells Kirk that nobody will question the death of an officer who had refused orders, and with the Captain's death everybody else will move up a rank. However, one of Chekov's men turns on him and helps Kirk out, allowing him to turn the tables as his own bodyguard arrives. Chekov is sent to an "agony booth", a torture device employed aboard the ship.

The group meet up again and assess their situation. Quizzing the computer they learn that a dimensional jump is possible under the conditions they experience, and that it may be reversible. Kirk investigates his own alter-ego's record, finding that this Kirk rose to command by killing Captain Pike and has since led the ship on a mission of conquest and destruction through the galaxy.

Back on the USS Enterprise, the mirror landing party are not faring so well. All reacted to their new surroundings with aggression, paranoia and arrogance, leading Spock to realise what had happened almost instantly and confine them to the brig.

On the ISS Enterprise, Scotty and McCoy work to alter their warp engines and transporter system to allow them to return home. Kirk is warned by Spock that he will be forced to move against him if he does not act against the Halkans, something Spock does not look forward to as Kirk's enemies have a habit of mysteriously vanishing. Kirk goes back to his quarters and finds Lieutenant Marlena Moreau there, a beautiful female officer who refers to herself as "the Captain's woman". It quickly becomes clear that the mirror Kirk had no great affection for Moreau, or her for him - the relationship was purely one of sex in exchange for rank and status. When Kirk declines to take advantage of the situation Moreau assumes that she is being dumped, immediately fearing that she will lose rank and openly contemplating who else might be willing to take her on. When Spock reports that he has been ordered to kill Kirk, Moreau reveals the "Tantalus field", an alien device in Kirk's possession. This device is capable of scanning any location on the ship, targeting any individual, and disintegrating them at the touch of a button. The mirror Kirk has used the device ruthlessly to eliminate any rivals or adversaries, but Kirk declines to use it to kill Spock.

Spock, meanwhile, is contemplating the suspicious behaviour of Kirk and quizzes the computer about the research they have undertaken. Realising that something strange must have happened he intercepts Kirk as he sets up the transporter to return them home, and takes him to sickbay at phaser point. Finding the rest of the landing party gathered there confirms his suspicions. The group manage to get the upper hand on Spock, knocking him out and badly injuring him.

Sulu arrives with some henchmen, happy to have a chance to kill both Kirk and Spock and thus gain the Captaincy for himself. Back in Kirk's quarters Moreau uses the Tantalus device to eliminate Sulu's group, and Kirk knocks him out. The group head for the transporter to beam back home, but McCoy stays behind to treat Spock's injury. Unfortunately Spock awakens and uses a mind meld to learn the truth of what has happened.

In the transporter room Moreau is waiting, determined to force Kirk to take her with them. Uhura disarms her, but the transporter is remotely disabled - Scotty can still activate it, but only if it is done manually from the console - meaning one of them must remain behind. Spock arrives and tells them that he is willing to operate the transporter and send them back to their own universe. In the remaining seconds Kirk tells Spock that logic must surely indicate that the Empire cannot continue as it is forever - eventually it will self destruct. Spock agrees, and Kirk suggests that he try to reform it, by starting by finding a reason to spare the Halkans that Starfleet will accept. He suggests that Spock take the Tantalus device from Kirk's quarters, which will allow him to protect himself, and that Moreau would make a good ally for him. Spock agrees to consider the proposal, and the landing party beam out.

Back on the Enterprise the officers reflect on their experience. Spock notes that it was far easier for civilised men to play the role of barbarians than it was for barbarians to play the role of civilised men, whilst McCoy notes that Spock seemed to fit in well in an uncivilised universe. Kirk is amused to find himself signing a report for an officer who has recently transferred to the Enterprise - one Lieutenant Marlena Moreau.

Analysis

One of the better and certainly more memorable Trek episodes, Mirror, Mirror is mostly fun for watching how the evil version of the Enterprise functions. It's all very well to aspire to the goody-goody Federation ideals but being bad can also be fun, and watching those who are normally good be bad can be delicious. As I say in my DS9 reviews, this is something that TOS and Enterprise really understood about the Mirror universe but DS9 didn't - the pleasure here comes from watching our normally heroic characters being selfish, self centered, sadistic, and generally unpleasant. There's a lot of fun to be had discovering that officers advance through assassination - an idea that would later be applied to the Klingons, though in different form - and little details like the fact that Kirk murdered Captain Pike to get command of the Enterprise.

There isn't any heavy handed moral here, really, though Spock's note that civilised men can act the barbarian whilst barbarians can't do the reverse is a telling one. But this isn't really a "message" episode, it's more about the fun to be had with the premise of the show.